One type of conventional separation is “dead-end” filtration. In dead-end filtration, a fluid containing suspended particles is directed along a flow path where it is forced to flow through a porous filter medium, such as a porous membrane. The primary characteristic of the membrane is its pore size distribution. Those particles which are relatively small compared to the pore size distribution may be transmitted through the membrane with the fluid while those particles which are relatively large compared to the pore size distribution may be retained on the surface or within the pores of the membrane, thus effecting a separation of the particles suspended in the fluid. The size above which most particles are retained and below which most particles are transmitted is referred to as the cut-off size of the membrane.
As particles accumulate within or on the surface of the porous medium (i.e., a process known as fouling), the effective sizes of the membrane pores decreases. This results in an increase in the power required to maintain the flow through the membrane and a shift in the cut-off size. Both of these results of fouling have important consequences for separation processes. The increase in power required increases the cost of the separation process and the shift in cut-off size affects the function of the separation processes. In many separation applications, especially those involving biological fluids, the shift in cut-off size renders the use of conventional dead-end filters ineffective.
Another type of conventional separation is “tangential” or “cross-flow” separation. Cross-flow separation can alleviate, and in some cases eliminate, the detrimental effects of fouling in dead-end filtration. In cross-flow separation, the mixture of particles and fluid is driven through a passage or channel, the walls of which include a porous medium, such as a porous membrane. One portion of the mixture, (i.e., the retentate or concentrate) passes tangentially along the membrane and exits the device without passing through the membrane while the remaining portion of the mixture (i.e., the permeate or filtrate) passes through the membrane to effect the separation. The purpose of forcing a portion of the flow to be parallel or tangential to the membrane surface is to generate a layer of high shear near the membrane surface which tends by various mechanisms to reduce the fouling that would occur in dead end filtration.
Although effective, conventional cross-flow separation is not without serious problems. One problem is non-uniform distribution of the flow over the membrane surface, especially with cross-flow devices that are made up of flat sheets of membranes with wide rectangular channels for the feed flow. Another problem, which affects both tubular and rectangular channel forms of conventional cross-flow devices, is that the shear rates generated in conventional cross-flow devices are frequently not large enough to prevent the development of a layer of highly concentrated suspended particles on the feed (or upstream) side of the membrane. This layer of concentrated particles is referred to as a gel layer and the phenomenon by which it is created is referred to as concentration polarization. The gel layer acts as a filter with much smaller pores than the membrane. If particles of a certain size are retained by the membrane, much smaller particles will be retained by the gel layer. As a result, many conventional cross flow devices and processes are unable to effect separations of particles that differ in size by less than an order of magnitude.
The gel layer may be significantly reduced or eliminated by increasing the shear rate. Shear rates, which in conventional cross-flow devices may be on the order of 104 inverse seconds, may be increased by increasing the pressure gradient between the feed inlet and the retentate outlet. However, it has generally been considered impractical to substantially increase the shear rate because a large pressure gradient from the feed inlet to the retentate outlet causes a large permeate flux at the inlet end of the device and a relatively small permeate flux at the retentate end. The large flux near the inlet counteracts the shear and leads to concentration polarization, while the small flux at the outlet reduces throughput and efficient use of the membrane. Restricting the permeate flow by means of a control valve downstream of the permeate outlet does not alleviate this problem. Restricting the permeate flow with a valve changes the pressure on the permeate side of the membrane uniformly but does not significantly change the large pressure gradient along the upstream side of the membrane. Consequently, the difference between the permeate flux near the inlet and the permeate flux near the outlet remains. It can even happen that the permeate flow is restricted so much that although the flux at the inlet end is reduced enough to avoid concentration polarization, the permeate flux at the outlet end is reversed and flows from the permeate side to the retentate side of the membrane. This phenomenon is known as Starling flow.
The gel layer may also be significantly reduced or eliminated by keeping the permeate flux below a critical value that depends on factors such as the shear rate, the membrane properties, and the suspension being separated. Control of permeate flux may be accomplished by control of transmembrane pressure (TMP), where TMP may be defined as the difference between the pressure at a location on the upstream side of the membrane and the pressure at the corresponding point on the downstream, or permeate, side of the membrane. The flux through the membrane tends to increase with TMP. However, the rate of fouling increases with flux; so the relationship between permeate flux and TMP is not generally linear. In conventional cross-flow devices, as the TMP is increased, the permeate flux increases, but the rate of increase approaches zero as the permeate flux asymptotically approaches a maximum, regardless of how much the TMP is increased.
A well known approach to controlling TMP is to re-circulate the permeate fluid through the permeate passages at a fast enough flow rate that the change in pressure within the permeate passages from the portion near the inlet end of the device to the portion near the retentate end of the device is the same as the corresponding pressure drop along the upstream side of the membrane. This approach, which is described in U.S. Pat. No. 4,105,547, requires the added expense of a pump to drive the re-circulating permeate flow. Another difficulty with re-circulating permeate to maintain uniform TMP is that generally the permeate volume flow rate is so much smaller than the retentate flow rate that the re-circulation rate must be very large, or the cross sectional area of the permeate passages must be excessively small, to establish a pressure drop equal to that on the retentate side.